Condensation vapor degreasers have been used for many years to remove oils, greases, and related organic soils from metal or plastic parts. Typically, a solvent is heated within a solvent chamber for producing hot vapors that engulf parts suspended within a cleaning chamber. The hot vapors condense onto the surface of the parts and dissolve the soils on their surfaces. The soils are thereby suspended in the condensed solvent and are returned by gravity to the solvent chamber.
One advantage of using solvent vapors for cleaning parts compared to submerging parts within a solvent bath is that the solvent vapors remain substantially free of the soils which become suspended in the solvent. The clean solvent vapors are distilled from the solvent soil solution. Other related advantages include the use of only a small amount of solvent to clean large numbers of parts and the use of elevated temperatures to accelerate the rate of soil dissolution into the solvent.
Well-known criteria are used to select solvents for vapor degreasing operations. First, the solvents should be effective for dissolving the type of soils to be cleaned. Second, the solvents should exhibit low specific heats and relatively low heats of vaporization so that energy requirements for raising the temperature of the solvents and for vaporizing the solvents are minimized. Third, the solvents should boil at the lowest temperatures which are effective for dissolving the soils so that the parts can be conveniently handled and energy expenditures are further minimized. Fourth, the solvent vapors should condense easily and dry quickly, leaving no residues. Finally, it has been a longstanding criteria to select solvents that exhibit relatively high flash points to avoid the possibility of igniting the vapors during the insertion or removal of parts from a cleaning chamber.
The most widely used solvents are halogenated hydrocarbon solvents. In particular, most solvents used in vapor degreasing operations are chlorinated or fluorinated hydrocarbons. Although these compounds exhibit very favorable characteristics for use as degreasing solvents, they are also known to be hazardous chemicals. The chlorinated hydrocarbons have been identified as a potential human carcinogen and the fluorinated hydrocarbons have been blamed for damaging the earth's ozone layer. Neither of these types of halogenated hydrocarbons is biodegradable, and they tend to persist in the environment as a toxin to fish and other wildlife.
Various attempts have been made to limit the exposure of operators and the environment: to these hazardous chemicals by modifying conventional vapor degreasers to reduce the amount of vapors which can escape from the cleaning chamber when parts are inserted or removed. For example, U.S. Pat. Nos. 4,029,517 and 4,101,340 to Rand disclose special apparatus for removing solvent vapors prior to opening the cleaning chamber. Of course, in addition to removing vapors already present in the cleaning chamber, it is necessary to prevent additional vapors from entering the heating chamber. Accordingly, the known apparatus also provides for shutting off the supply of heat to the solvent chamber and for isolating the cleaning chamber from the solvent chamber.
Although the known apparatus significantly reduces vapor losses from the cleaning chamber, the process of removing cleaned parts and inserting new ones is very time consuming because the cleaning chamber must remain closed until the solvent has cooled beneath its boiling point. The solvent must also be reheated every time a new part is added, and this takes additional time and energy to revaporize the solvent. Of course, even if such time-consuming procedures are used to minimize vapor losses during the degreasing process, special care is still required for adding or removing the solvent from the degreaser, and for storing and disposing of the solvent after use.
Other solvent compounds, particularly hydrocarbon solvents, are known to exhibit good cleaning qualities and to pose less long-term risk to human health and the environment, but they also exhibit detectable flash points. In other words, the known hydrocarbon solvents pose a significant safety hazard in the form of a risk of explosion and fire when present in only limited concentrations in the air. Accordingly, the hydrocarbon solvents are seldom used in vapor degreasing operations; and in the limited instances in which they are used, they are contained in special concentrations with other substances which minimize the potential of the vaporized solvent to explode.